1. Field of the Invention
The present invention relates to a rectangular nickel-metal hydride secondary cell.
A rectangular nickel-metal hydride secondary cell permits an energy density higher than that of a rectangular nickel-cadmium cell and, thus, is used widely nowadays. Because of the high energy density achieved by the rectangular nickel-metal hydride secondary cell, it is necessary to ensure an excellent air-tightness even in the event of heat generation caused by, for example, over-charging.
In general, the rectangular nickel-metal hydride secondary cell comprises a rectangular cylindrical metal case having a bottom. The open peripheral portion of the metal case is inwardly folded to form a folded portion. Also, an inwardly projecting stepped portion is formed below the folded portion of the metal case. An electrode group is housed in the metal case. The electrode group is constructed such that a nickel positive electrode and a hydrogen absorption alloy negative electrode are alternately superposed one upon the other with a separator interposed between the adjacent nickel electrode and the hydrogen absorption alloy electrode. An alkali electrolyte is also contained in the metal case. The secondary cell further comprises a rectangular sealing plate, which is fixed to the metal case at the position with an insulating gasket between the folded portion and the stepped portion, and the insulating gasket formed of a synthetic resin and positioned along the inner wall of the metal case. The gasket is interposed under a compressed state between the omen end portion of the metal case and the periphery of the sealing plate so as to hold the peripheral portion of the sealing plate.
In the secondary cell of the construction described above, the sealing plate and the insulating gasket are fixed to the open end portion of the metal case, as follows. In the first step, the insulating gasket holding the sealing plate is mounted on a stepped portion along the inner surface of the metal case. Then, the open end portion of the metal case is pressed to reduce the diameter of the metal case in this portion, followed by inwardly folding the open end portion of the metal case so as to fix the sealing plate together with the insulating gasket to the inner surface of the metal case in a region near the open end portion.
As described above, an operation for reducing the diameter in the open end portion of the metal case is included in the process. What should be noted is that the material of the metal case in the shrunk portion is distributed so as to be concentrated on the four corner portions in the open end portion of the metal case, which is rectangular in cross section. Naturally, the corner portion is rendered higher than the central portion of the side positioned between two adjacent corner portions. When the open end portion having the particular shape is inwardly folded, the folding angle of the corner portion is rendered greater than that in the central portion of the side, giving rise to a problem that the air-tightness of the secondary cell is changed depending on the folding angle of the corner portion. To be more specific, if the folding angle of the corner portion is increased in an attempt to improve the air-tightness, an excessive force is applied to the corner portion of the metal case. Therefore, the metal case occurs deformation such as deflection or strain. On the other hand, if the folding angle of the corner portion is diminished in an attempt to prevent the metal case from being deformed, the air-tightness of the secondary cell is markedly lowered.